用金相显微镜和扫描电镜观察了FV520B钢焊接接头母材、焊缝及热影响区的微观组织；并通过透射电镜给出了母材和焊缝的微观形貌。结果表明：母材组织为细小均匀的回火马氏体、弥散分布的析出相及高密度位错，而焊缝为粗大的板条马氏体及少量的析出相，说明母材力学性能优于焊缝。利用疲劳实验获得了高平均拉应力下接头的疲劳强度及S-N曲线等。通过对疲劳断口的观察，发现疲劳断裂主要有两种形式：破坏于焊趾处和焊接缺陷处，由于这里严重的应力集中，加速了疲劳裂纹的萌生。
Microstructure of base metal, weld seam and heat-affected zone of welded joints of FV520B steel were examined by optical microscopy, SEM and TEM. The results show that the mierostructure of the base metal eonsists of fine tempered martensite and dispersively precipitated phase particles with high-density dislocations, whereas the microstructure of the weld seam is composed of coarse lath martensite and few of precipitated phase particles, indicating the better mechanical properties of the base metal. Fatigue tests were carried out to obtain the fatigue strength and the S-N curve of the welded joints subjected to a high mean stress. The observation of the fatigue fracture surface shows that fatigue failure occurs at the weld toe and welding defects due to serious stress concentration of these locations accelerating fatigue crack initiation.
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